Ever since lubricating oils were prepared from crude oils, refiners have experienced difficulty with congealation of these products at low temperatures. Part of the difficulty arises from a natural stiffening at low temperatures of the hydrocarbons comprising the bulk of the oil. This type of congealation can be corrected quite easily by the use of a solvent such as kerosene to reduce the viscosity of the oil. The remainder of the difficulty arises from the crystallization at low temperatures of the paraffin wax present in almost all heavy mineral oil fractions. Upon crystallization, the paraffin wax tends to form interlocking networks which adsorb oil and form a voluminous gel-like structure which restricts the flow or "pour" of the oil. Even though refining processes known as dewaxing have been developed to remove most of the paraffin from lubricating oil fractions, the small amount of wax remaining after dewaxing can cause serious problems. Even such small amounts of wax can raise by tens of degrees Fahrenheit the temperature at which an oil will flow freely as measured by a suitable "pour point" test. Since removal of the last traces of wax from oils is a difficult and costly matter, other answers have been sought by refiners.
Various pour point depressants have been developed and those to reach the commercial market have primarily been organic polymers, although some monomeric substances such as tetra (long chain alkyl) silicates, phenyl tristearyloxysilane, and pentaerythritol tetrastearate have been shown to be effective. Presently available commercial pour point depressants are believed to be represented by the following types of polymeric materials: polymethacrylates, for example, copolymers of various chain length alkyl methacrylates (see, for example, U.S. Pat. No. 2,655,479); polyacrylamides (see, for example, U.S. Pat. No. 2,387,501); Friedel-Crafts condensation products of chlorinated paraffin wax with naphthalene (see, for example, U.S. Pat. Nos. 1,815,022 and 2,015,748); Friedel-Crafts condensation products of chlorinated paraffin wax with phenol (see, for example, U.S. Pat. No. 2,191,498); and vinyl carboxylate, such as dialkyl fumarate copolymers (see, for example, U.S. Pat. Nos. 2,666,746; 2,721,877 and 2,721,878).
Esters of maleic anhydride/alpha-olefin copolymers have been suggested as pour point depressants. For example, U.S. Pat. No. 2,977,334 describes the use of copolymers of maleic anhydride and ethylene which are esterified with low or high molecular weight alcohols and/or amidized with an amine. These resins are described as being useful as pour point modifiers, gelling agents, thickeners, viscosity improvers, etc., for mineral and synthetic oils including functional fluids and lubricating oils. U.S. Pat. No. 2,992,987 describes a class of lubricant additives useful as pour point depressants which are ethylene-maleic anhydride copolymers esterified to 80% or more, preferably 90-100%, with a mixture of straight-chain saturated hydrocarbon alcohols having from 8 to 24 carbon atoms. The unesterified carboxylic groups can be left unreacted or can be reacted with such materials as ethylene or propylene oxide alcohol esters, or lower-dialkylamino-lower-alkyleneamines. U.S. Pat. Nos. 3,329,658 and 3,449,250 describes copolymers of maleic anhydride and alpha-olefins such as ethylene, propylene, isobutylene or styrene as being useful dispersancy and detergency additives for oils, as well as pour point depressants and viscosity index improvers. The copolymer is esterified to about 30 to about 95% with aliphatic alcohols or mixtures of alcohols having from 10 to 20 carbon atoms, and the remaining carboxyl groups are reacted with an amine of the following formula ##STR1## where R.sub.1 and R.sub.2 are selected from the group consisting of aliphatic hydrocarbon radicals having from 1 to 4 carbon atoms and the cyclohexyl radical, R.sub.3 is an aliphatic hydrocarbon radical having from 2 to 4 carbon atoms, and R.sub.4 is selected from the class consisting of hydrocarbon atom and aliphatic hydrocarbon radicals having from 1 to 4 carbon atoms.
U.S. Pat. Nos. 3,702,300 and 3,933,761 describe carboxy-containing interpolymers in which some of the carboxy radicals are esterified and the remaining carboxy radicals are neutralized by reaction with a polyamino compound having one primary or secondary amino group and at least one mono-functional amino group, and indicate that such interpolymers are useful as viscosity index improving and anti-sludge agents in lubricating compositions and fuels. The patentee indicates that it is critical that the mixed esters described in these patents include both relatively high molecular weight carboxylic ester groups having at least eight aliphatic carbon atoms in the ester radical and relatively low molecular weight carboxylic ester groups having no more than seven aliphatic carbon atoms in the ester radical.
Although many pour point depressants have been suggested and many are available in the market, concerted efforts are constantly being made to find new pour point depressants which are more economical and more effective than the depressants heretofore known in the art. In particular, a great deal of interest exists in pour point depressants which are capable of imparting other desirable properties to the lubricating compositions to which they are added in addition to pour point depressant properties.